Electro-optical image production



July )23, 1940 J. R. HEFELE` ELECTRO-OPTICAL IMAGE PRODUCTION 2, 1936 2 Sheets-Sheet 1 EEO:

Fjzled Sept.

July 23, T940. J, R, HEFELE 2,208,927

ELEGTRO-OEFII'ICL IMAGE PRODUCTION Fixed sept. 2, 195e 2 sheetssneez. 2

' i i 2 AMI? 26 i 25T l 42 "E FIG. 3

A In /14 a A TTOR/VEV Patented July 23, 1940 iiiiiif ELECTRO-OPTICAL IMAGE PRODUCTION John R. Hefele, Brooklyn, N. Y., assigner to Bell This'iinvention Irelates toV electro-optical image production and particularly to' apparatus for and `4method of producing television images.

Anfobject'oxthiswinvention is to improve the quality-of television; images by increasing` the accuracy'withlwhich the light tone values of the `field orvieware reproduced in the images thereof.

@In'fthe television system shown and described l herein-.forfthelurpose of illustrating this inven` tiongfthere is'produceda unidirectional image current havingamplitude variations correspondingitofthe `tone values of successively scanned elemental areasof` afield of view and having l otherfamplitude` Variations "produced as the result of periodically reducing the currentl to a value equal to or preferably less than that producedwwhenvscanning an area `of black tone value, Suchffa reduction .in amplitude may be 2U introducedtat theendwof `each scanning line of a field of View, as, for example, by periodically interrupting-j the scanning of the `field `of view, andgtherefore, the light `activation of the light sensitive element `employed in scanning.y The electrical transmittingapparatus upon which thisunidirectionalimage current is impressed suppresses `thedirect vcomponent` which is representativefof the averageatone` Value of the iield beinsscanned and-which varies in amplitude S0 as` theaveragemtone Value of the eld changes. The amplitude of the steady component of the unidirectional imagecurrent, which component isrepresentative,of; the average tone value over a given.,period; varies ,in accordance with the .'25 amplitude otrtheimpulses set `up as the result of,..periodically interrupting thescanning. In `the,alternating image=current, the amplitude of the impulses produced as the result of periodically interruptingmthe scanning, measured be- 40 tween .the-peaksof' the impulses and zero,` varies ina'ccordancefwith `the 'amplitude of the sup-` `system is provided `in which a source of image electromotive force,.the direct component of which has been suppressed, is fconnectedto. the input of one -oi' a plurality of amplifier stages of 55 y a, ,televisions current transmission circuit and in rent ilowing through the blue `and `Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York iii-@Application September 2, 1936, Serial No. 99,022 s claims.` (cl. 17g- 7.3)

parallel therewith to a circuit for correcting for the suppression of the direct current component, the output of the correcting circuit being conneet/ed to the television transmission circuit at a position subsequent to the rst stage.

In a preferred embodiment, the television electromotive force is supplied to an amplifier stage of the corrector circuit througha small condenser. The output of the amplier is connected to a self-biased detector `which selects the peaks of the .impulses representing black tone values. A mechanical relay controlled by the detected current controls the charging of a condenser in the input circuit of the final ampliiier stage of the television circuit to control the average currentin the output circuit of the amplifier.`

Other arrangements for `correcting for the suppression of the direct and low frequency components of the television image current, which components are representative vof the average tone value of the ield of view at the transmitting station may be used. Several of these arrangements are described in `greater' detail hereinafter.

While the correction for the suppression of the direct component is important inall television systems to improve the quality of the received image, it is especially necessary in color work. For example, assume a three-color system having red, green, and blue components such as is shown in Patent 2,037,166 issued April 14, 1936, to J; R. Hefele. object is red, the green and blue lamps at the receiver would still glow with average brightness unless means such as the circuits to be hereinafter described are provided to reduce the cur green lamps to a negligible amount.

The invention will be more readily understood from the following description taken in connection with the accompanying drawings forming a part thereof in which: i

- Figi is a diagrammatic View of a television system, including apparatus for inserting a direct component into an image current which corre vsponds to that which `has been suppressed at the transmitting station; i u

. Fig. 2 is a detail showing of a portion of the scanning apparatus of Fig. 1; and

Figs. 3 and 4 are diagrammatic views of other arrangements for correcting for the suppression ofthe direct component Voigthe image current generated at the television transmitting-station.

Referring now to Fig. l, there is shown, by way of example,v a television transmitter vT and aitelevision receiver R connected by a suitable If `the elemental area of the line L. The transmitter comprises a source of light I3 of constant intensity associated with an optical system, including a disc II provided with a row of apertures i2 arranged in a spiral line and adapted to be rotated by a motor i3, and a lens system, shown by Way of example as a lens Ui. Rotation of the disc il causes light from the source l to pass through the successive' apertures E2 to thereby produce a moving beam of light which is directed by means of the optical system Id to illuminate successively the elemental areas ci the iield of view O. The disc H is rotate-d at a speed such that, `for one complete revolution, elemental areas of parallel adjacent lin-es of the complete field of view O are successively illuminated within the period of persistence of vision. Light reliected from the successive elemental areas of the field of View O reaches a light sensitive electric device I5, which is thereby activated to produce an image current varying in accordance with the tone values of such elemental areas. In front of the disc li is a screen iii cf opaque material provided with an opening Il which determines the size of the field O. The opening il' is so dimensioned that light passes through the apertures l2, one at a time, to the field of View O For the purpose of the present invention, the scanning apparatus operates in such a manner that the scanning of the field of View is periodically interrupted for a brief interval one or more times during each complete scanning period. In a preferred arrangement for effecting this result, the lateral distance between the apertures I2 of the disc I I is made greater than the width of the opening I'I in the screen I6, by an amount equal to the width of one or a few elemental areas, as shown in Fig. 2.

The light sensitive device l5, supplied with energizing current from the direct current source I8, will be activated, under control of the above described optical system, to produce a unidirectional image current. This unidirectional image current consists of a slowly varying direct component the amplitude of which corresponds to the average of the tonev values or brightness of the field of view O, and alternating components of different frequencies, respectively, the amplitudes of which are determined by the successively scanned elemental areas of the eld of view. The frequencies of these alternating components may extend from a frequency of one million cycles or more, to a low frequency which is approximately the frequency of scanning of the entire field of view, i. e., around twenty cycles per second. When no light is applied to the light sensitive device due to the interruption of the scanning light beam, for example, the amplitude of the current supplied `by the light sensitive device I5 will be reduced to a low value, substantially Zero, for example.

While the major portion of the image current band may be satisfactorily transmitted over wellknown communication circuits, for example, tele-A phone lines or coaxial cables, it is well known that such transmission circuits include apparatus which suppresses or greatly attenuates the direct and very low frequency components of a television signal.

The electromotive force across a resistive element I 3U in circuitwith the light sensitive devicevI5 is applied to a circuit including a coupling condenser C and the input circuit of an ampliler thermionic tube I9 which includes a resistor I3I of relatively high resistance and a biasing battery |32. The coupling condenser C suppresses the direct current component of the unidirectional image current. The tube I9 is connected through additional stages of amplication and transformer 2l, to a line L. There is thus impressed upon the line L an alternating image current including the correction impulses which are of larger amplitude than other portions of the image current and of a polarity the same as that produced by scanning black elemental areas. In this respect, the present invention is similar to the invention in an application of W. A. Knoop filed Sept. 2, 1936, Serial No. 99,023. Diierent means are used in the present invention, however, to correct for the suppression of the direct current component. In the circuits of thisvinvention, there is no possibility of oscillations being set up due to feedback through the correcting circuit.

At the receiving station the line L is connected through a transformer to a suitable amplifying device 22 which may comprise a plurality of stages, and the output circuit of the amplifier 22 is connected through the condenser 24 to the input electrodes of an electron discharge device 23. The customary biasing battery 25 and resistor 25 are used in the circuit between the grid 21 and the cathode 28 of the electron discharge device 23. The output circuit of the device 23 includes a source of plate potential and an output resistor 36. 'I'he output of the tube 23 is connected to the input circuit of a second electron discharge device 29 by means of a circuit including a coupling condenser 4 I The circuit between the cathode 33 and the grid 34 of the device 29 consists of a biasing battery 30, a resistor 3| and a condenser 32 connected in series, a portion of resistor 58 being shunted across this condenser. The output circuit of the tube 29 includes a source of plate potential 31, resistor 38, thyrite resistor 39 and neon lamp 4U. The purpose of the thyrite resistor 39 and the ballast resistor 38 in connection with the operation of the neon lamp 40, is to produce a straight line relationship between the current iiowing through the lamp and the thyrite element in series and the electromotive force across the lamp and thyrite. For a more complete description of the purpose and method of operation of these elements, reference may be made to the above-mentioned Knoop application.

Signal currents after being amplified by the device 22 are also applied to the input circuit of anelectron discharge device 50 by means of a circuit including a coupling condenser 42 of re1- atively low capacity so that it will present a high impedance to low frequencies but permit the relis a self-biased detector which selects the peaks of the impulses representing black tone values. The signals applied to the input circuit of the detector 50 are so phased by properly selecting the number of stages in the ampliers 22 that an increase in amplitude of the correcting impulses produces an increase in current in the h output circuit of the device. This tends to increase the negative bias on the grid and hence decrease the output current. Finally a state of equilibrium is attained between the correcting signals and the grid bias potential of dll the detector. Shuntng the resistance `52 is the condenser 53 which is adapted to integrate or4 by the battery 55 so that the electron discharge device 56 will amplify the smoothed voltage impressed across its input circuit without distortion. The output circuit of the tube 56 comprises a source of plate potential 51 and a resistor 58, aportion of which is connected across the condenser 32. The voltage to which condenser 32 becomes charged, due to the voltage drop across the portion of resistance 59 connected in shunt with condenser 32, thus controls the output current of the tube 29 in accordance with the value of the average of the tone values of the scanned elemental line at the transmitting station.

The amplified alternating image current and the direct current component, both of which are present in the output circuit of the tube 29, are applied `to the glow lamp 49. With no picture signals (the correcting signals thus being of minimum amplitude) the biasing battery 39 is ad` Justed so that the dark signal level for the neon lamp is produced. If the voltage across the condenser 32 is lowered (because of the action of the corrector circuit in responding to pulses of greater amplitude), the brightness of the lamp 40 rises with the increase in amplitude of the correcting pulses. The increased impulse decreases the bias on the tube 29, thus increasing n the output of this tube and decreasing the output of tube 5B. This lowers the blason tube 29 and increases the current through the neon lamp 49. If a pulse is generated which is of smaller amplitude than this last pulse, the bias on the tube 29 will be increased and the brightness of the lamp reduced.

Associated withthe lamp i9 is a scanning disc 60 provided with a row of apertures 9i arranged in a spiral line and driven by a motor 62 in synchronism and in phase with the scanning disc il at the transmitting station. Any suitable system for maintainingthe discs l I and 99 in synchronism may be used. In one such system, which is herein described by way of example, alternating current of substantially constant frequency is produced by a vacuum tube oscillator 93 which is located at the transmitting station for controlling the operation of the motor speed control circuit 94 at that station. Current from this source is also transmitted over line 65 to thereceiving station' R for controlling the operation of motor speed control means` 66 at thatstaticn.l The motors i3 and -62 of both stations are thus maintained in synchronism. `Suitable ampliers tl' and 93 at the transmitting and receiving stations, respectively, are used to amplify the current generated bythe oscillator 63. For a more complete de-` scription of the synchronizing system herein briefly described, reference maybe made to U. S. Patent 1,999,376 of H. M. Stoller issued April 30, 1935.

An observer at E may view the image produced bythe cooperation of the receiver scanning means 6l) `and the light source 49. through an opening 69 in an opaque screen 19. Glow lamp 49 may be of any desired type adapted to supply light which varies with variations in amplitude of the applied image current. A suitable lamp for this purpose is -disclosed in U. S. Patent 1,918,309 of H. W. Weinhart, issued July 8, 1933. As therein described, the glow lampmay` include a charge.

comprising a rare gas and a small percentage of active gas, asdisclosed in U. S. Patent 1,871,266

of F. Gray, issued August 9, 1932.

- Fig. 3 shows another circuit for'borrecting for the suppression of the direct component ofthe image current generated at the transmitting sta- Instead of the amplified and filtered i lili` lainp i9 and the thyrite resistor 39. An advani tage of this arrangement is that the alternating current amplifying tube 29 is not affected by changes in direct current from the correcting circuit. Thus the amplifying action or gain of the tube 29 is independent of the correcting cirv cuit.

In this circuit, the output of the tube 29 isr impressed by means of a circuit including a coupling condenser 8i (which prevents any direct current from the output circuit of the device fromi reaching the output circuit of the power tube Z9) upon the image producing circuit which comprises the neon lamp 40, the thyrite resistor 39 and the ballast resistor 38 (which also serves as a load resistor in the output circuit of the tube 99). A direct current source 31 applies plate potential for the tube 29 and resistor 84 is included in series with the source of potential. The out-` put circuit of the power tube 89 includes an inductor S9 (for` preventing any alternating current .i

from the power tube 29 from reaching the tube 80), the Aimage producing neon lamp 40, thyrite resistor 39, ballast resistor 38, andsource of plate potential 82. The output of the correcting circuit thus is used to vary the brightness of the neon lamp 99 directly instead of through the tube 29 which now serves as the output tube of the image signal circuit alone rather than as the output tube for both image signal circuit and correcting circuit as shown in Fig. 1.` `With the exception of this diiTerence, the description and method of operation of this circuit are similar to those above i described in connection with Fig. 1; i

Still another circuit, which may be considere l a preferred arrangement, for correcting for the suppression of the direct component of a tele# vision current at the transmitting station is shown in Fig. 4. In this arrangement, the amplifying means for the alternating component is similar to the corresponding part of the circuit' shown in Fig. l, power tube 29 serving as the out-` put tube for the `alternating component. By means of the condenser 32 inserted in the input circuit of the power tube 29, the tube 29 also serves as the output tube for the direct com-` ponent.

The television electromotive force after being amplified by the device 22 and applied to the input circuit of the alternating component amplifying tube 23, is also applied through a suitable coupling condenser 42 of` small capacity to the input circuit of an amplifying device 90 which serves to isolate the input of the correcting circuit from the normal image signal amplier, i. e., from the input circuit of tube 23. The circuit between the grid 93 and the cathode 94 of the deI vice 99 includes a biasing battery 9| and resistor. Heating current for the cathode of tube 99 is obtained by means of a suitable transformer 95; the primary of which is shown connected` to an. alternatingl current generator 96. The output of s the tube 90 is coupled by means of a transformer 1.

liliv 91 to the grid 98 of the self-biased detector tube 99through a potentiometer resistor IBG. A source of plate patential lul is also connected in the circuit between the plate |02 and the cathode 94 of l; the isolating tubeSil. In the output circuit of the self-biased detector 99 is a source of plate potential |03 and a load resistor |04, which is tapped atanintermediate point thereof to supply selfbias through the Yresistor lill] to the grid 98. A condenser |05 connected across the part of resistor A|04 used to supply grid bias may be used to smooth the feedback impulses. Condenser |96 connected vacross the output circuit of the selfbiased detector Q9 serves to integrate and smooth '15' the impulses produced in the output circuit of the device `99. The purpose and method of operation of the self-biaseddetector 5l@ are similar tothose of the selfebiased detector 5u of Fig. 1 and reference may be made to the description zi given above in connection with that figure.

The detected and Asmoothed signals are applied through a smoothing filter circuit, generally represented by a resistor lul and a condenser Hi8, to the input circuit of a direct current amplifying device IUS. A biasing battery HG with its positive terminal connected to the grid of the tube |09 isl used in the event that the detected and integrated signals are too negative. This battery I I9, and similarly the batteries 55 oi Figs. .zl and 3, may be omitted in some cases, depending. on the intensity of the signal and on the type of tube. The device l amplifles the smoothed voltage impressed across its input circuit without distortion.

The output circuit of the tube lug comprises a source of plate potential IEE and winding i i3 of an electromagnetic relay lili. The space currentoi the tube H39 passes through the winding H3 `of the relay HL2 in such a direction as to tend to bias the relay tongue ||5 to the le-it or spacing contact S. The relay tongue is vibrating continuously because it is driven by 60-cycle alternating current Afrom the transformer 95 through the winding l |55 of the relay I lli. When t no space current flows through the tube lillfl, the tongue H5 of the relay i3 is biased to the right or marking Contact M by the biasing current through the winding H9, the regulating resistance Ill and the variable resistor H8. When e the average space' current through the tube |09 equals the biasing current through the winding H9, the direct current voltage across the condenser 32 is half the voltage of the battery |22 which is connected between the spacing contact S and the marking contact' M of the relay lili through resistors |26 and |2l. This potential is applied across condenser 32 through lter cirsuit '|23 comprising inductors |255, resistors |25, and condensers |26.

The operation of the circuit shown in Fig. 4 is as follows: Assume an increase in amplitude of the signal applied to the input circuit of tube 90, representing a correcting impulse of large amplitude at the transmitter, the impulse being 65`iso poled that the grid is swung positive. The

output of the tube 9E) is thus increased, thus decreasing the output of tube 99, reducingl the negative bias on tube |09 and thus increasing the output of this'tube. As the average space i'fcurrent'from the amplifying device U9 increases, the direct component correcting voltage across the con'denser 32 decreases and vice versa. As the relay ||4 serves as an amplifier it may be replaced by any other equivalent amplifier. It 75:' has the advantage in'this oase that the signals are'amplified Without their phase being reversed but this condition is also true if an even number of stages of electron discharge amplifiers are used.

As a modification the condenser 32 may be placed in the input circuit of another power tube in a manner similar to the circuit of Fig. 3. As a further modification, the self-biasing detector 98 may be replaced by a gas-filled tube. In this latter case the circuit elements are not changed but the constants may be. fications may be made in the invention as disclosed, the scope of which is indicated by the appended claims. v

What is claimed is:

l. In a television system a source of television image electromotive force, the direct component of which has been suppressed and having recurring impulses of predominant amplitude andA said anode current flowing through said. lightl producing device being controlled in accordance with the amplitude of said unidirectional electromotive force impressed upon said control electrode-cathode circuit, and means for impressing said television image electromotive force upon the circuit including said light producing device for further controlling the current through said light producing device in accordance with the variations of said television image electromotive force.

2. In a television system in accordance with Various other modi# CTI claim l in which an amplier having a plurality of stages of amplification is provided for ampli-v fying the television image electromotive force, in which the output of said ampliiier is connected to the circuit including said light producing device, and in which the means for producing a unidirectional electromotive force corresponding to the suppressed component is connected to the input of the first of said stages through a condenser of small capacitance, such that it will have a high impedance to frequencies lower than the fundamental frequency of the predominant amplitude impulses of the television image electromotive force but a low impedance to said fundamental frequency.

3. In a television system, a source of television image electromotive force, the direct component of which has been suppressed and having recurring impulses of predominant amplitude and of a predetermined polarity, the amplitude of which impulses is a measure of the suppressed direct component, a current path, means comprising a iirst electric discharge device under control of said source of television image electromotive force, means for connecting the output circuit of said first electric discharge device r across said current path to produce across said path variations. of electromotive force corresponding to the variations of said source of television image electromotive force, a second electric discharge device, means for controlling the input circuit of said second electric discharge device in accordance with the amplitude of the recurring impulses of said television image electromotive force, and means for connecting the, output circuit of said second electric discharge device across said current path to produce across said path an electromotive force corresponding to the suppressed direct component of said television image electromotive force, the means for connecting the output circuit of the rst electric discharge device across said vcurrent path comprising a capacitive element, and the means for connecting the output circuit of said second electric discharge device across said current path comprising an` inductive element.

JOI-IN R. HEFELE. 

